The objective of this project is to directly characterize the binding interaction etween the major gastrointestinal hormones (gastrin, cholecystokinin and secretin) and their receptors. This will be done by measuring binding of 125I-labelled hormones to a plasma membrane receptor preparation using competitive binding techniques, and by the measurement of relative effects of hormones on plasma membrane cyclic nucleotidases. BIBLIOGRAPHIC REFERENCES: Baur, Sonia and Bacon, Virginia C. A Specific Gastrin Receptor on Plasma Membranes of Antral Smooth Muscle. Biochem Biophys Res Commun 73:928, 1976. Bacon, V.C. and Baur, S. Interaction of Gastrin with Plasma Membrane Receptor: Biochemical Characterization. (Abstract) Gastroenterology 70:A-97, 1976.